The Energy Budget of Thinking

At the spatial cognition workshop I mentioned in the previous post, we were asked to think big picture thoughts about what we most wanted to find out about spatial cognition. My big wish is to be able to find out the energy costs, literally the calorie expenditure, of various thought processes.

This notion is not totally far fetched. Functional magnetic resonance imaging (fMRI), the brain imaging technology that shows specific regions of the brain "lit up" when they are being used, is a measure of blood flow. Blood flows in order to bring oxygen. Oxygen comes in order to support respiration. And respiration occurs in order to generate energy. So at least conceptually, the increase in blood flow that occurs when a brain region is active could be a proxy for energy demand.

Workshop participant and neuroscientist Anjan Chatterjee explained to me some of the substantial difficulties in converting the time-averaged relative measure of fMRI output into a quantitative measure–although he did seem intrigued by my questions. But I recall that all of the seafloor and subseafloor imaging techniques that we use in marine geophysics, including seismic reflection profiling and side-looking sonar, began as relative measures, displaying the relative loudness of sonar echoes as varying shades of non-quantitative uncalibrated greyness. After several decades of refinement, such data can now be processed to yield quantitative estimates of attributes of the materials being imaged, such as density and elastic modulus. So I remain optimistic about fMRI or its successor technologies eventually answering my question.

So why would I want this information, anyway? Because thinking takes a lot of energy. The brain is only approximately 2% of the body's mass, but consumes approximately 20% of its energy. For a species that evolved in an energy-limited environment, it would be evolutionarily advantageous to evolve ways of thinking efficiently, that is to say ways of thinking that conserve energy relative to other ways of thinking.

In casual conversation, some kinds of thinking are referred to as "hard work." In professional discourse among cognitive scientists, the same idea is heard, but they use the phrase "cognitive effort," for example to assert that one problem-solving strategy requires more or less cognitive effort than another. Mental rotation is a task that is said to be cognitively demanding; that's the ability tested by those exercises in which you are asked to decide whether a pictured item is the same as an example item except for having been rotated. In Mary Hegarty's talk at the workshop, she showed that on a standard test of mental rotation some people substitute non-rotational strategies such as counting the number of cubes or checking to see if the end limbs of the figure are parallel or perpendicular to each other. These substitute methods don't work for all items, but where they do work they are said to save cognitive effort.

There is a whole set of human decision-making processes called "heuristics," which are rules of thumb for decision-making that are "fast and frugal" (Goldstein & Gigerenzer (2004). The "fast" part is clear–these heuristics save time. But what about the "frugal" part? Frugal means "thrifty" or "careful with resources." What resources are we talking about here? It seems to me that "frugal" can be understood, at least in part, as "conservative of energy," literally energy as produced by respiration and measured in calories.

The reason that I want to know the energy budget of thinking in calories is because sometimes there is a trade off between exerting yourself physically and exerting yourself mentally. For example, imagine you go for a meandering walk around the countryside, for example, hunting or gathering or shopping or doing field geology. After you finish your tasks, you want to return to your starting point. One option is to retrace your steps, using landmarks that you observed on your outbound journey. But it is also possible, for some people at least, to cut cross-country directly towards back towards your starting point.

The kind of knowledge that allows you to retrace your steps using observed landmarks is called "route knowledge." The kind of map-like knowledge that lets you know the direction back towards your starting point is called "survey knowledge." Some people have the ability to assemble survey knowledge by passing through a terrain without ever seeing a map of that terrain, as shown in a study by Ishikawa & Montello (2006), summarized here. People vary widely on their ability to assemble survey knowledge in this way, and many people find it quite difficult.

It costs more physical energy to retrace one's path along the convoluted outbound route than to take the shorter straight line back to the starting point. But it costs mental energy to assemble the survey knowledge that will allow one to take the shorter direct path. To understand what has been gained and lost in such a trade off, we need to know both expenditures in the same currency, calories for example. Exercise physiologists could tell us about the calories expended on the straight path versus the meandering path. Now we need to know about the calories expended in the mental effort to assemble survey knowledge from route observations.